1. Field of the Invention
This invention relates to the field of electronic circuit design, and in particular to a CMOS current generator that provides an output current that is insensitive to process parameters, specifically, insensitive to threshold voltage.
2. Description of Related Art
Conventional current sources are dependent upon a number of factors, including temperature, voltage, and process variations. Bandgap voltage references can provide a stable voltage, and PTAT (proportional-to-absolute-temperature) circuits can be used to compensate for current changes with temperature. Process variations generally affect the threshold voltage Vt of the fabricated transistors, and a variety of techniques have been developed for generating a current that is insensitive to the threshold voltage Vt.
FIGS. 1 and 2 illustrate example circuit diagrams of conventional constant-current sources 100 and 200, respectively. In FIG. 1, a bandgap voltage typically provides a reference voltage Vref, and a high gain operational amplifier controls the current through an NMOS transistor so as to maintain this reference voltage across a resistor R. This controlled current will be equal to Vref/R, and will be independent of Vt. As is known in the art, however, the fabrication of a resistor can result in a variance of resistance of as much as 40%. FIG. 2 illustrates a current source 200 that does not rely upon a reference voltage, per se, and is commonly referred to as a xe2x80x9cbeta multiplier referenced self-biasing current sourcexe2x80x9d. The output current of the current source 200 is independent of Vt, but it is dependent upon the resistance value of a resistor R1, which, as noted above, is difficult to control. Often, the resistor R1 is trimmed after fabrication to provide the desired output current, but this is generally a costly process step.
U.S. Pat. No. 5,469,111, xe2x80x9cCIRCUIT FOR GENERATING A PROCESS VARIATION INSENSITIVE REFERENCE BIAS CURRENTxe2x80x9d, issued Nov. 21, 1995 to Kwok-F Chiu, presents a current generator wherein the Vt-independent reference current is based on the difference between two base-emitter voltages of two bipolar transistors, and does not depend upon a controlled resistance value.
Japanese patent JP 1-263706, xe2x80x9cBIAS CIRCUITxe2x80x9d, issued 20, Oct. 1989 to Daijiro Inami, presents a current generator wherein the Vt-independent reference current is based on maintaining a particular relationship of sizes of N-MOS transistors, and also does not depend upon a controlled resistance.
It is an object of this invention to provide a current source that is independent of process variations, particularly variations in threshold voltage, Vt. It is a further object of this invention to provide a Vt-independent current source that can be embodied using MOS technology.
These objects and others are achieved by a current-compensating circuit that provides compensation to a reference voltage such that the current through a diode-connected MOS transistor remains constant, regardless of threshold voltage. The compensating circuit includes another MOS transistor that is connected as a voltage follower in saturation. Variations in the component of the reference voltage that are produced by the effects of process variation on this other MOS transistor act to correct the current variations that these same process variations cause in the diode-connected MOS transistor.